A variety of applications of such mid-infrared light are expected in the fields of gas analysis, medical care, environmental measurement, and the like. However, they have not been realized since there exist no semiconductor light-emitting devices emitting infrared light in this range. In the present situation, not only light-emitting devices but also any appropriate light-receiving devices do not exist. A photodiode for optical communication has an InP (indium phosphide) substrate and an InGaAs (indium gallium arsenide) light-receiving layer and is sensible to 1.3 μm-1.6 μm near infrared light.
However, there is no appropriate material having an absorption edge of 1.6 μm or longer, so that it is impossible to fabricate a device receiving infrared light of 1.6 μm or longer. FIG. 4 schematically depicts a band structure formed in a semiconductor bulk crystal and shows a valence band and a conduction band of a light-receiving layer. The valence band and the conduction band extend uniformly. A forbidden layer lies therebetween. The energy width of the forbidden band, that is, the difference of energy (energy level) between the valence band and the conduction band is band gap Eg. The light-receiving layer absorbs light thereby exciting an electron in the valence band to the conduction band. If a pn junction is fabricated with an absorptive material, the excited electrons, holes are separated to form photocurrent, which is taken out to the outside. A photodiode which senses incidence of light is thus structured.
However, only light having energy of band gap Eg or greater is absorbed by semiconductor. Energy of light is given by hv=hc/λ. Here, h is Planck's constant, c is the speed of light in vacuum, λ is wavelength, and v is frequency. When energy of light E=hc/λ is expressed by eV and wavelength λ, is expressed by μm, the relation of λ=1.2398/E holds. The band gap energy of the substrate InP is 1.3 eV, and thus the cutoff wavelength is 0.95 μm. The InP substrate absorbs light with the shorter wavelength. Since light with the longer wavelength is allowed to pass through, the substrate can be used for a device measuring light of 0.95 μm or longer. A thin film of Group III-V matched to the InP substrate can be formed thereon thereby to fabricate a light-receiving device.
However, the lattice-matching to the substrate is required as a condition. Ternary InGaAs is often used as a light-receiving layer. Under the condition of matching to InP, a composition of In0.53Ga0.47As is used. This has an absorption edge wavelength of about 1.6 μm. Therefore, it is suited for a photodiode receiving light for optical communication, such as 1.3 μm or 1.55 μm.
However, there is no appropriate photodiode capable of receiving light of the longer wavelength, 2 μm-3 μm. In order to receive 2.5 μm, the band gap energy has to be approximately Eg=0.5 eV, However, there are few appropriate semiconductors having such a narrow band gap, and if any, it is impossible to fabricate a pn junction to form a photodiode.
Japanese Patent Laying-Open No. 09-219563 (Patent Document 1) proposes Group III-V semiconductor including Sb (antimony) as a laser material adapted to mid-infrared light, but it states that Group III-V crystal including Sb is difficult to form and is too unstable to fabricate a practical device. Japanese Patent Laying-Open No. 09-219563, which denies Sb-based Group III-V, insists that Group III-V compound semiconductor including nitrogen, such as GaInNAs (gallium indium nitride arsenide) or InNPAs (indium nitride phosphide arsenide), matched to a GaAs substrate or an InP substrate may be a crystal having a band gap energy of 0.73 eV or less and it is possible to form a light-emitting device emitting mid-infrared light of 1.7 μm-5 μm and a light-receiving device receiving with such a wavelength range. Energy of 0.73 eV corresponds to a wavelength of 1.7 μm. Japanese Patent Laying-Open No. 09-219563 insists that since the electronegativity of nitrogen is extremely high, as the lattice constant decreases, the band gap also decreases, as long as the nitrogen proportion is small.
On the other hand, GaInAs (gallium indium arsenide) which has been used in optical communication is characterized in that as the lattice constant increases, the band gap decreases. Thus, GaInNAs which is made of GaInAs with the addition of nitrogen can have a bandgap decreased with the lattice constant matched to the InP or GaAs (gallium arsenide) substrate, so that it is possible to fabricate a mixed crystal having a band gap energy of 0.73 eV or less.
Japanese Patent Laying-Open No. 09-219563 describes a structure of a semiconductor laser with emphasis on a light-emitting device. A description of a photodiode is only made in Example 1. The photodiode structure in Example 1 in Japanese Patent Laying-Open No. 09-219563 is illustrated as follows, from the top in order.
p-GaInAs cap layer (0.2 μm thick)
p-InP layer (1.0 μm thick)
non-doped GaInNAs strain-free light absorption layer (0.5 μm thick)
n-InP layer (1.0 μm thick)
n-InP substrate
Japanese Patent Laying-Open No. 09-219563, however, does not mention the mixed crystal ratio of the GaInNAs strain-free light absorption layer. The value of the band gap of the GaInNAs strain-free light absorption layer is unknown. Moreover, no mention is made of whether it can receive light of 1.7 μm or not. The request for examination was not made for Japanese Patent Laying-Open No. 09-219563 and the application is deemed to be withdrawn. In order to lower the band gap energy to 0.6 eV (corresponding to wavelength 2 μm), the proportion of nitrogen N has to be made considerably large (about 10%). However, it is impossible to grow a GaInNAs thin film having such a high nitrogen proportion on the InP substrate. Group V element has an extremely high vapor pressure at a growth temperature, which makes vapor deposition difficult. In addition, in this case, two different Group V elements exist. The vapor pressures are considerably different, and successful growth is impossible even with MBE (Molecular Beam Epitaxy).
Japanese Patent Laying-Open No. 2003-282927 (Patent Document 2) proposes a photodiode in which an In0.53Ga0.47As layer having a lattice constant smaller than that of InP and an In0.55Ga0.45As layer having a lattice constant larger than that of InP are grown in an overlapping manner as light-receiving layers on an InP substrate, and an InP cap layer is grown thereon. Two layers each having a lattice constant greater or smaller than that of InP are combined so that the average lattice constant agrees with the lattice constant of InP. The absorption wavelength of the former In0.53Ga0.47As is λg=1.57 μm, and the absorption edge wavelength of the latter In0.55Ga0.45As is 4=1.63 μm. Therefore, infrared light of up to 1.63 μm can be received. As the conventional photodiode for optical communication has an In0.53Ga0.47As light-receiving layer with the absorption wavelength of λg=1.57 μm, light can be sensed to the infrared side by 0.06 μm as compared with the conventional one.
However, a semiconductor lattice-matched to InP or GaAs and having a narrow band gap hardly exists. Then, two types of structure having a superlattice as a light-receiving layer have been proposed. It is known that there are two different kinds of junctions of a mixed crystal semiconductor thin film layer matched to an InP substrate. The normal one is called type 1 and a newly discovered junction is called type 2. The difference in manner of these two kinds of junctions will be described.
FIG. 5 shows a band of a superlattice structure of type 1. When a heterojunction is formed between a material A having a narrow bandgap EgA and a material B having a wide band gap EgB, conduction band CB of B lies above conduction band CA of A, and valence band VB of B lies below valence band VA of A. This is referred to as a type 1 heterojunction.
Although band gap EgA of material A and band gap EgB of material B are determined, the amounts of the conduction band difference (CA−CB) and the valence band difference (VA−VB) in the junction are not easily known. This may be a difference of work function φ, which is not known in advance. It is not determined until a sample is prepared. The heights of the conduction band and the valence band of material A are given by CA, VA, respectively. The heights of the conduction band and the valence band of material B are given by CB, VB, respectively.CA−VA=EgA  (1)CB−VB=EgB  (2)where EgA, EgB are constants. If CA−CB or VA−VB is known, all of CA, VA, CB, VB are determined. However, neither CA−CB nor VA−VB is easily known, and the manner of the band created with the heterojunction of material A and material B is not known in advance. This is because CA−CB is not determined until the heterojunction is fabricated. However, even if CA−CB is determined, the means for finding the values of CA, VA, CR, VB is limited. There is no means for directly detecting how the junction is formed.
In a type 1 heterojunction, when EgA<EgB, CA−CB<0 and VA−VB>0. In general, (CA−CB)(VA−VB)<0 can be defined. The difference of conduction band heights and the difference of valence band heights are opposite. In the normal heterojunction, when a valence band goes down, a conduction band goes up, and such a relation is the most common.
In a superlattice structure having materials A, B repeatedly laid in layers as well as in a single heterojunction, the relation of bands is the same. The single junction of type 1 is as shown in FIG. 5. An example of EgA<EgB is shown. CB>CA, VA>VB. The conduction band difference WAB=CA−CB is negative. The valence band difference UAB=VA−VB is positive. In type 1, such a simple expression as WABUAB<0 can be made. A number of type 1 junctions are alternately laid as shown in FIG. 9.
Absorption of light with a long wavelength in material A causes transition as shown by p, creating an electron-hole pair in material A. Absorption of light with a short wavelength in material B causes transition as shown by q, creating an electron-hole pair in material B. This depends on a wavelength (energy) of light to be absorbed. Transition which goes across the junction as shown by r occurs with a low probability. When monochrome light with low energy is applied, only transition of p occurs. Given that hc/λ is photon energy, light with a wavelength λ, longer than hc/EgA is not absorbed. A photodiode having a superlattice including this multi junction as a light-receiving layer cannot detect light with energy smaller than narrow band gap EgA.
For example, when a superlattice of InGaAs/InP is fabricated, a type 1 junction is fabricated in which InP corresponds to material B having a wide band gap and InGaAs corresponds to material A having a narrow band gap. Type 1 is the normal junction. The junction of most semiconductors is type 1. For detection of infrared light having a long wavelength, there is no other choice but to use a semiconductor material having a narrow band gap. However, a semiconductor having such a narrow band gap hardly exists.
However, the junction as shown in FIG. 6 may be contemplated. In the heterojunction between material D and material G, conduction band bottom energy CD, CG and valence band peak energy VD, VG change similarly. In other words, in this junction, WGD=CD−CG>0, UGD=VD−VG>0 or vice versa. In general, (CD−CG)(VD−VG)>0.
In Non-Patent Document 4 (Klem et al.), a superlattice thin film structure having a heterojunction of GaAs0.5Sb0.5/In0.53Ga0.47As was formed on an InP substrate by MBE, and with various combinations of film thicknesses of the superlattice, photoluminescence (PL) of the GaAsSb/InGaAs superlattice structure at 77 K was examined. The angle dependency of (004) X-ray diffraction intensity was also measured. The appropriate mixed crystal ratio is determined by the condition of lattice match since the structure was formed on an InP substrate.
The mixed crystal ratio of InGaAs is In=0.53, Ga=0.47, as repeatedly described so far, which is most commonly used as a light-receiving layer on the InP substrate. This light-receiving layer is that of the photodiode for optical communication with 1.2 μm-1.6 μm, which has already been used widely. The band gap energy is Eg=789 meV and the absorption edge wavelength is 1.57 μm. Photoluminescence corresponding to band-to-band transition 1.57 μm appears.
The other GaAsSb is a material difficult to prepare. The growth speed of GaAsSb is slow. The mixed crystal ratio of GaAsSb is unstable. When GaAsSb is manufactured, the mixed crystal ratio varies widely depending on the composition or temperature of the material gas.
The band gap WGD, UGD in the GaAsSb/InGaAs junction is also unknown. With three kinds of combinations of superlattice film thicknesses of 16.5 nm/16.5 nm, 8.5 nm/8.5 nm, 5.5 nm/5.5 nm, photoluminescence at 10 K was measured. The result was that photoluminescence had a peak of 0.45 eV, 0.48 eV, 0.53 eV, respectively, and the photoluminescence intensity was greater in the combination of a thinner superlattice film thickness.
If this is a type 2 junction and photoluminescence is generated by transition between the conduction band of InGaAs and the valence band of GaAsSb, given that the energy difference between the bottom of the InGaAs conduction band and the peak of the GaAsSb valence band is Q, energy Eph of the photoluminescence should be:Eph=Q+(h2/8)(1/t2me+1/s2mh)where h is Planck's constant, s, t are the thicknesses of one rising layer, one falling layer, respectively, and me, mh are the effective mass of electron hole. With Q and s, t varied, possible values of Eph can be calculated. With s=t=16.5 nm, 8.5 nm, 5.5 nm, the respective three kinds of superlattices were formed, and the measured photoluminescence was Eph=0.45, 0.49, 0.54 eV, respectively. Based on the measurement results, Non-Patent Document 4 estimated that the intersection band gap energy Q=0.43 eV. Non-Patent Document 4 estimated that the band gap energy of GaAsSb (As:Sb=50:50) was EgD=800 meV and the band gap energy of InGaAs was EgG=810 meV.
As shown in FIG. 6, since EgG=Q+UGD, EgD=WGD+Q, it can be calculated that UGD=380 meV, WGD=370 meV. The offset WGD of the conduction band is finally calculated here. WGD is the most important parameter of the type 2 junction. When this is determined, the intersection band gap is also found, for the band gap is known. Non-Patent Document 4 insists that Q=430 meV.
Non-Patent Document 4 assumed the intersection band gap energy CD−CG=0.37 eV. The rising layer is D=GaAsSb, and the falling layer is G=InGaAs. Although the mixed crystal ratio of GaAsSb varies depending on the material gas Group V/Group III ratio, when 5.5, it is calculated that the absorption edge is 1.55 μm and the band gap energy EgD=0.800 eV. On the hand, InGaAs is a well-known mixed crystal with In of 53% and Ga of 47%. The absorption edge wavelength is 1.57 μm and the band gap energy is EgG=0.789 eV. On the assumption of CD−CG=0.37 eV, the gap of the valence band should be VD−VG=0.359 eV. FIG. 7 is a band junction diagram based on such an assumption. Non-Patent Document 4 estimates such a band diagram based on the measurement of photoluminescence.
However, different values of intersection band gap Q have been reported until then. Non-Patent Document 5 mentions the intersection band gap energy Q=230 meV. Non-Patent Document 6 says the intersection band gap energy Q=250 meV.
The value of Q varies depending on the target to be measured and the assumption. A number of assumptions have existed for the value of the intersection band gap Q.
This illustrates the unstableness of GaAsSb and the difficulty of evaluation. Even if the band gap EgG, EgD unique to a material cannot be made small, if the intersection band gap is small, this intersection transition leads to a possibility of fabricating a photodiode capable of receiving infrared light with a long wavelength.
The type 2 junction is complicated and the terms are thus defined provisionally here. In the type 2 junction, the layer in which both a conduction band and a valence band become high is called a rising layer D. A layer in which both a conduction band and a valence band become low is called a falling layer G. WGD is referred to as a conduction band offset. UGD is referred to as a valence band offset. The relation EgG=Q+UGD, EgD=WGD+Q holds between the band gap EgG of the falling layer, the band gap EgD of the rising layer and the conduction band offset, the valence band offset. Q is the degree of energy of mid-infrared light (2-3 μm).
Photoluminescence corresponding to Q is generated. It is assumed that this is due to electron transition between the conduction band of the falling layer and the valence band of the rising layer. The generated photoluminescence may be used to form a light-emitting device for mid-infrared. An attempt has been made to fabricate a light-emitting device for mid-infrared using the GaAsSb/InGaAs junction. By contrast, the generation of photoluminescence leads to the expectation that a light-receiving device absorbing mid-infrared light to generate photocurrent may be formed. In other words, a photodiode for mid-infrared light may possibly be fabricated using the type 2 junction.
In Non-Patent Document 3 (Yamamoto et al.), In0.53Ga0.47As/GaAs0.5Sb0.5 single junction (SH; single-hetero: not MQW) was grown on an InP substrate by MBE with the temperature varied (505° C., 515° C., 530° C.) and with the Group VIII ratio varied (the Group VIII ratio=5.5, 12, 18), and with application of Ar (argon) laser (514 nm), the growth temperature dependency of photoluminescence was examined. Here, photoluminescence is approximately from 1.5 μm to 1.63 μm and is not the photoluminescence (2.3 μm) resulting from the intersection band gap.
In addition to photoluminescence corresponding to the band-edge wavelength of 1.57 μm, photoluminescence having a peak at the lower energy 1.61 μm appeared in the GaAsSb layer grown with the material gas Group V/III ratio of 5.5. When the growth temperature is increased from 505° C. to 530° C., the peak of 1.61 μm decreases. It was mentioned that emission of 1.61 μm could not be detected in a sample with the growth temperature of 530° C. This does not correspond to either the band gap of InGaAs or the band gap of GaAsSb. This may be emission resulting from impurity interlevel transition. In Non-Patent Document 3, the conduction band energy difference is assumed to be WGD=370 meV, and assuming a potential in InGaAs (falling layer G), a Schrödinger equation is formulated and solved self-consistently.
The first orbital energy of the conduction band of the falling layer In0.53Ga0.47As is E1=124 meV, the second orbital energy is E2=150 meV, and Fermi energy is Ef=176 meV. This is shown in FIG. 8. Although the first orbital and second orbital energy of the valence band of the rising layer is not calculated, it is believed that E1=50 meV, approximately.
Since the energy difference between the bottom of the conduction band of the falling layer and the peak of the valence band of the rising layer is Q=370 meV, the energy released to the peak of the rising layer (GaAsSb) by an electron dropping from the first orbit of the conduction band of the falling layer to the first orbit of the valence band of the rising layer is 544 meV. This corresponds to a wavelength 2.46 μm. It is assumed that this is the source of photoluminescence of 2.4 μm-2.5 μm. In other words, Q=WGD=370 meV was confirmed, according to Non-Patent Document 3.
Non-Patent Document 2 (Rubin et al.) proposes a photodiode in which an In0.53Ga0.47As buffer layer is formed on an InP substrate, a quantum well (total thickness of 1500 nm) of 150 pairs of InGaAs/GaAsSb of 5 nm/5 nm is grown thereon, and in addition, In0.53Ga0.47As window layer is provided. It says that light of 2.23 μm was received at room temperature. This is the first photodiode using the type 2 junction.    Patent Document 1: Japanese Patent Laying-Open No. 09-219563    Patent Document 2: Japanese Patent Laying-Open No. 2003-282927    Non-Patent Document 1: T. Murakami et al., “InxGa1-xAs/InAsyP1-y detector for near infrared (1-2.6 μm),” Conference Proceedings of Indium Phosphide and Related Materials    Non-Patent Document 2: Rubin Sidhu, Ning Duan, Joe C. Cambell & Archie L. Holmes, “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells,” IEEE Photonics Technology Letters, Vol. 17, No. 12, pp 2715-2717, December 2005    Non-Patent Document 3: A. Yamamoto, Y. Kawamura, H. Naito & N. Inoue, “Optical properties of GaAs0.5Sb0.5 and In0.53Ga0.47As/GaAs0.5Sb0.5 type II single hetero-structures lattice-matched to InP substrates grown by molecular beam epitaxy,” Journal of Crystal Growth, Vol. 201/202, pp 872-876, May 1999    Non-Patent Document 4: J. F. Klem, S. R. Kurtz and A. Datye, “Growth and properties of GaAsSb/InGaAs superlattices on InP,” Journal of Crystal Growth vol. 111, pp 628-632, 1991    Non-Patent Document 5: G. A. Sai-Halasz et al., Solid State Commun. 27, p 935, 1978    Non-Patent Document 6: Y. Sugiyama et al., Journal of Crystal Growth vol. 95, p 363, 1989